Known optical smoke and other particle detectors comprise a source whose light is directed on a path or beam which may be interrupted by smoke or other particles or media which scatter light from the directed path to a sensing photocell, the photocell responding to the scattered light by producing an electrical signal.
It is also well known to compensate for variations in the light source by employing a second photocell exposed to light direct from the source and connected in a circuit with the first photocell so that the response of the second cell to undesired light source variations cancels the effect of such variations on the first , smoke sensing, cell as disclosed for example in U.S. Pat. Nos. 2,301,367 and 3,409,885.
The object of the present invention, however, is to employ a second photocell in a way distinctly different from that described above in that the second cell is not exposed to light directly from the source, but rather the second cell is disposed so that it compensates not for light source variations, but instead compensates for the background light conditions in the dark chamber of a smoke detector, for example. These light background conditions are meant to include outside ambient light leaking into the chamber and such internal source light as is scattered from dark chamber walls outside the directed path from the source. The background light condition affects the total response of the smoke sensing photocell unduly when the background light sensed approaches the energy of the light scattered by smoke from the light beam or path. That is, the background light tends to mask the smoke scattered light and reduce the sensitivity of the detector to very low densities of smoke which occur early in a fire.
Thus the objects of the present invention are to increase the sensitivity of a particle detector to small changes in particle density, to permit reduction in the source light intensity, and to compensate for increase in background scattered light as dust and the like accumulate on the walls of the dark chamber.